Using wild-type flies and mutants selected for resistance to halothane, we explored the relationship between general anesthesia and receptors for neurotransmitters. We found that the mutants were moderately sensitive to cholinergic drugs but not to other neurotropic agents. Moreover, flies selected for resistance to a GABA agonist are normally sensitive to halothane. Genetic mapping of two halothane-resistant mutants proves that they affect loci that are distinct from each other and from that of two previously mapped mutants. Thus, resistance to halothane can be effected by several different genes. Finally, the phenomenon of anesthetic cut-off has been evaluated in Drosophilia with homologous series of n-alkanes and n-alcohols.